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fix differenceInDays across DST (#533) #1630

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Apr 9, 2020
Merged

fix differenceInDays across DST (#533) #1630

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992fa97
Fix differenceInDays across DST (#533)
dcousens Feb 4, 2020
43cff14
move 86400000 to constant MILLISECONDS_IN_DAY
dcousens Mar 30, 2020
1edbcac
add description explaining DST behaviour
dcousens Mar 31, 2020
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16 changes: 4 additions & 12 deletions src/differenceInDays/index.js
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Original file line number Diff line number Diff line change
@@ -1,6 +1,4 @@
import toDate from '../toDate/index.js'
import differenceInCalendarDays from '../differenceInCalendarDays/index.js'
import compareAsc from '../compareAsc/index.js'
import requiredArgs from '../_lib/requiredArgs/index.js'

/**
Expand Down Expand Up @@ -42,15 +40,9 @@ export default function differenceInDays(dirtyDateLeft, dirtyDateRight) {
var dateLeft = toDate(dirtyDateLeft)
var dateRight = toDate(dirtyDateRight)

var sign = compareAsc(dateLeft, dateRight)
var difference = Math.abs(differenceInCalendarDays(dateLeft, dateRight))
var result = (dateLeft - dateRight) / 86400000
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Please move 86400000 into a var MILLISECONDS_IN_DAY

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@dkozickis moved


dateLeft.setDate(dateLeft.getDate() - sign * difference)

// Math.abs(diff in full days - diff in calendar days) === 1 if last calendar day is not full
// If so, result must be decreased by 1 in absolute value
var isLastDayNotFull = compareAsc(dateLeft, dateRight) === -sign
var result = sign * (difference - isLastDayNotFull)
// Prevent negative zero
return result === 0 ? 0 : result
// round towards zero
if (result > 0) return Math.floor(result)
return Math.ceil(result)
}
10 changes: 10 additions & 0 deletions src/differenceInDays/test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -62,6 +62,16 @@ describe('differenceInDays', function() {
assert(result === 0)
})

it('ignores any localtime differences with DST', function() {
const a = new Date('2019-10-05T15:00:00.000Z') // for TZ=Australia/Sydney, this is 1 hour before DST
const b = new Date('2019-10-06T14:00:00.000Z') // for TZ=Australia/Sydney, this is 1 day after DST
const c = new Date('2019-10-07T14:00:00.000Z') // for TZ=Australia/Sydney, this is 2 days after DST

assert(differenceInDays(c, b) === 1)
assert(differenceInDays(b, a) === 0) // 23 hours < 1 day
assert(differenceInDays(c, a) === 1) // 47 hours < 2 days
})

it('does not return -0 when the given dates are the same', () => {
function isNegativeZero(x) {
return x === 0 && 1 / x < 0
Expand Down